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Steven Simmons

Technical Services Manager Southwest District

Common Explained

When you’re still green with experience or if you just entered an HVAC apprenticeship, there will be terms or sayings that you will not understand.  One of these things will be the term “common” when discussing wiring. I’m going to cover what common is and give examples.

The word Common in the HVAC field is used everywhere, it can be almost anything. Almost every electrical device has a common. Common for each device is the same BUT its meaning is only local. What I mean by that is: the definition for what “common” is, changes with each item or device.

Confused yet? Let’s break down some items within the system for a better understanding:

Single Phase Motors & Capacitor

schematic diagram
*Wiring diagrams highlighted with colors chosen for high contrast- not for wire color designation*

For the motors, I have highlighted the L1 & L2 power wires through a basic Carrier 25HCB condenser unit as they feed the compressor and condenser fan motors. The fan motor’s terminals are not labeled but are the same as the compressor’s. I circled the “commons” in purple. For the motors, there are two windings within the motors: Run to C and Start to C. The two windings come together at a “common” terminal as this meeting spot is what they both have in common.

For the capacitor in the center—the capacitor has one input and two outputs. One is (F)Fan and the other is (H)Hermetic (compressor). These two outputs have the same terminal for their input- common.

Relay Contacts

Relay contact terminals are labeled (C) Common, (NO) Normally Open, (NC) Normally Closed. When the relay is not energized C will be closed to NC and C open to NO. When the relay is energized, both will be reversed. Now it will be C closed to NO and C open to NC. Each terminal (NO & NC) have a relatable contact: (C) Common.

Catching on? Let’s discuss a more complicated “Common”:

Carrier Residential Packaged Unit Low Voltage

common wiring schematic
*Wiring diagrams highlighted with colors chosen for high contrast- not for wire color designation*

Low voltage starts in the red box at the transformer, 24vac goes to the Indoor Fan Board (IFB), through a fuse and on to the thermostat. Depending on what the thermostat calls for, 24vac will be sent down a different wire(s) to turn on/off the multiple devices in the system. The other low voltage wire (C) has an unbroken path to EVERY component in the unit. I have highlighted the pathway in blue and circled each component in purple. This wire/path is something each device has in “common”—it is the common wire. In layman’s terms, the common wire is the return pathway for voltage to come back to the transformer since it left out on the 24vac side.

The Common wire/terminal is always the one thing that is relatable or in common with the surrounding wires/terminals. Relays, motors, control boards, and even transformers all have some type of common wire/terminal. Common for a relay isn’t the same as common on a motor, but within each given device they carry the same meaning. Now go back and reread the second paragraph above—it will have a deeper meaning now.

Challenge: Check out the high voltage side of the Transformer. It has a COM terminal…. it’s not low voltage common. What is it?

That’s all.

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